1. Field of the Invention
The present invention relates to semiconductor switching devices and more particularly to thyristors.
2. Description of the Prior Art
In order to produce thyristors having high values of di/dt a form of construction is required which allows the firing process to spread rapidly. Among other things, the spreading of the firing process is determined by the position and form of the thyristor's control electrode.
In German Pat. No. 1,234,326, a thyristor is described which has a strip-like control electrode laying in a strip-shaped recess in the emitter electrode. The control electrode and the recess in the emitter electrode each have two parallel edges. In addition, they are arranged so that the control electrode is positioned at a first constant distance from the emitter zone and a second constant distance from the edge of the recess in the emitter electrode.
Experiments carried out with thyristors in which the control electrode is a constant distance from the edge of the emitter have shown that firing sets-in mainly at specific locations determined by the crystal orientation. Initially, these locations are very small causing a concentration of load current under certain conditions. The resulting overheating can lead to the destruction of the thyristor.
FIG. 1 illustrates the specific locations where firing occurs for the case where a semiconductor body 9 was made from a semiconductor crystal rod drawn from the melt in the &lt;111&gt; direction. A control electrode 8 contacts the semiconductor body 9 in a ring-shaped interior base portion 2 which is bordered by an emitter portion 1. The border edge 12 of the emitter 1 intersects the surface of the semiconductor body 9 in circular shaped line. Experiments using infrared imaging show that concentrated firing takes place at specific locations 14 which lie approximately parallel to the [011], [110] and [101] directions.